The present invention relates to a roll press including at least three rolls which form several press nips such as may be used, for instance, for dewatering or smoothing a fiber web, like a paper web. The invention particularly concerns profiling of the rolls and application of the roll shell supports for optimizing application of pressure in the nips.
The roll press with which the invention is used typically includes a shoe press type roll having a stationary support, a flexible roll shell which rotates around the support and a concave shoe or shoes along the support. The shoe press roll cooperates with a backing roll to define a press nip between the flexible roll shell and the backing roll at the concave shoe. The backing roll may sag between its end supports due to its weight and to the pressure applied to it by the press roll.
A third roll may cooperate with the backing roll to counter the sag. If the backing roll cooperates with a third roll which is not sag controlled, as shown in WO 93/12289, which corresponds to U.S. Pat. No. 5,404,811, difficulties can occur, due to roll sag, in the formation of a uniform press nip which is to be as straight as possible.
Although it is possible to provide the backing roll or the third roll with support elements in the press plane for controlling the sag of the shells of those rolls, this requires considerable expense. Furthermore, considerable thermal problems can be produced in the customary hydraulic support elements if the backing roll and/or the third roll are heated so as to improve the web dewatering or smoothing.